We report on the first pointed X-ray observations with XMM-Newton and RXTE satellites of the X-ray source
RX J0757.0+6306 = HT Cam. We detect a strong 515 s X-ray modulation
confirming the optical photometric period found in 1998, which definitively assigns
this source to the intermediate polar class of magnetic cataclysmic
variables. The lack of orbital sidebands in the X-rays indicates that
the X-ray period is the spin period of the accreting white
dwarf. Simultaneous ultraviolet and optical B-band photometry acquired with the XMM-Newton Optical Monitor and coordinated optical UBVRI photometric data acquired at the Nordic
Optical Telescope (La Palma) show that the optical pulse is in phase with
the X-rays and hence originates in the magnetically-confined accretion
flow. The lack of ultraviolet spin
modulation suggests that accretion-induced heating on the white dwarf
surface is not important in this source. Spectral analyses of
XMM-Newton EPIC and RGS data show that HT Cam has a multi-temperature spectrum and,
contrary to most intermediate polars,
it does not suffer from strong absorption. With its 86 min orbital
period, HT Cam is the third confirmed system of this
class below the 2–3 h period gap accreting at a low rate.

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